It is well known that many organic dyestuffs and dye intermediates precipitate from solution as amorphous solids, gums, or tars, rather than in crystalline form. In such cases it is often necessary to induce crystallization of the compound or to inhibit formation of the gummy or tarry product. With difficulty crystallizable compounds, it is sometimes necessary to resort to a combination of treatments including seeding with particles, mechanical agitation and shock. Routinely, an azo dyestuff may be extracted with a solvent which does not appreciably dissolve the crystallizand but which may dissolve out certain impurities, give an interface effect or cause polymorphic transformation. Treatment may include digestion with boiling solvent under reflux or extraction in a Soxhlet type apparatus followed by appropriate supercooling, or in long shaking with the poor solvent in the cold. Sometimes contact with the poor solvent for several days at room temperature is effective. Alternatively the solution may be subjected to a rubbing or shaking action for several hours. Crystallization sets in, probably due to the formation of nuclei. Typically bentonite, kieselguhr, silica gel or charcoal may be added to aid crystallization.
The manner in which a dyestuff is formed and the refining process by which the product formed is crystallized and purified may result in the product having varied physical properties. Variations in physical properties can render a dye crystallized by one procedure excellent for a specific use and the same dye crystallized in another manner, far less satisfactory for the purpose.
Disperse Azo dyes are formed directly from water-soluble reactants which result in an essentially water-insoluble dye. The anthraquinone dyes are formed in an organic solvent in which the dye may be soluble. Where the dye is soluble in the organic solvent, the reaction mass is `drowned` in a liquid medium in which the dye is insoluble, such as in water, a salt solution or an acid solution. The specific composition of the liquid medium is not as important as its properties, which are that the liquid medium be miscible with the solvent and that the solute be relatively insoluble in it.
However, before crystals can grow, it is known that there must exist in the solution a number of minute bodies called "centers of crystallization" or "nuclei", but how a crystal nucleus is formed is not known with any degree of certainty. Different systems require different degrees of supersolubility or super-cooling before nucleation will occur, and often the conditions required by one particular system vary according to its state.
Probably the best method for inducing crystallization is to inoculate or seed the supersaturated solution with small regular-sized particles consisting of the material to be crystallized. The seeds should be dispersed uniformly throughout the solution by means of gentle agitation. (Chemical Engineering Practice, Cremer & David, Vol. 6 pg. 405). In the instant invention, the solution is seeded with inert particles of a water-wettable polytetrafluoroethylene resin, which unexpectedly gives better results than seeding with particles of material to be crystallized. Seeding with inert particles effectively nucleates the solution to yield a dye in which the resin is occluded, and at the same time, obviates the time-consuming process steps conventionally utilized to induce crystallization of organic dyestuffs and dye intermediates which crystallize with difficulty.